Conventionally, an electric power steering device is known as a device for assisting a driver's steering operation by transmitting output torque of an electric motor to a steering shaft, a rack bar or the like of a steering mechanism. During a vehicle is running, when large force is input from a tire to a steering mechanism, for example, when a tire strikes on a curb, a wheel is steered and a large axial tension acts on a rack bar. Consequently, the rack bar moves in its axial direction and the steering shaft connected to the rack bar rotates. A state that, as described above, a reverse input is acting on a steering mechanism from a tire and a wheel is steered is referred to as a reverse input state. When the reverse input is large, a rack-end member disposed at the end of the rack bar strikes on a stopper member formed in a rack housing and impact force acts on the steering mechanism.
Therefore, for example, the electric power steering device proposed in Japanese Patent Application Laid-Open (kokai) No. 6-8839 is configured such that turning force of an electric motor is transmitted to a steering mechanism via clutch and inertia torque of the electric motor is not transmitted to the steering mechanism by letting the clutch out when a reverse input state is detected. The detection of the reverse input state is performed by comparing the steering speed detected by steering speed sensor with a preset value. Namely, when the steering speed exceeds the preset value, the state is judged as a reverse input state.